If this error occurred when using MEF in a Silverlight application, please ensure that the CopyLocal property of the reference to the MefExtensions assembly is set to true in the main application/shell and false in all other assemblies.

Error was: The package downloaded successfully but an error occurred while reading the contents of the package. See the inner exception for more details.. ---> System.InvalidOperationException: The package downloaded successfully but an error occurred while reading the contents of the package. See the inner exception for more details. ---> System.Reflection.ReflectionTypeLoadException: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information. at System.Reflection.RuntimeModule.GetTypes(RuntimeModule module) at System.Reflection.RuntimeModule.GetTypes() at System.Reflection.Assembly.GetTypes() at System.ComponentModel.Composition.Hosting.AssemblyCatalog.get_InnerCatalog() at System.ComponentModel.Composition.Hosting.AssemblyCatalog.get_Parts() at System.ComponentModel.Composition.Hosting.DeploymentCatalog.DiscoverParts(IEnumerable`1 assemblies) at System.ComponentModel.Composition.Hosting.DeploymentCatalog.HandleOpenReadCompleted(Object sender, OpenReadCompletedEventArgs e) --- End of inner exception stack trace --- --- End of inner exception stack trace --- at Microsoft.Practices.Prism.Modularity.ModuleManager.HandleModuleTypeLoadingError(ModuleInfo moduleInfo, Exception exception) at Microsoft.Practices.Prism.Modularity.ModuleManager.IModuleTypeLoader_LoadModuleCompleted(Object sender, LoadModuleCompletedEventArgs e) at Microsoft.Practices.Prism.MefExtensions.Modularity.MefXapModuleTypeLoader.RaiseLoadModuleCompleted(LoadModuleCompletedEventArgs e) at Microsoft.Practices.Prism.MefExtensions.Modularity.MefXapModuleTypeLoader.RaiseLoadModuleCompleted(ModuleInfo moduleInfo, Exception error) at Microsoft.Practices.Prism.MefExtensions.Modularity.MefXapModuleTypeLoader.HandleDownloadCompleted(DeploymentCatalog deploymentCatalog, AsyncCompletedEventArgs e) at Microsoft.Practices.Prism.MefExtensions.Modularity.MefXapModuleTypeLoader.DeploymentCatalog_DownloadCompleted(Object sender, AsyncCompletedEventArgs e) at System.ComponentModel.Composition.Hosting.DeploymentCatalog.OnDownloadCompleted(AsyncCompletedEventArgs e) at System.ComponentModel.Composition.Hosting.DeploymentCatalog.HandleOpenReadCompleted(Object sender, OpenReadCompletedEventArgs e) at System.Net.WebClient.OnOpenReadCompleted(OpenReadCompletedEventArgs e) at System.Net.WebClient.OpenReadOperationCompleted(Object arg)

The exception you're experiencing might be because you're setting the Region Manager as an import, and using it in the Initialize method, where it might have not been resolved.

One possible workaround for that would be to make use of constructor injection, so as to make sure that the import of type IRegionManager has been resolved at the time you call the
RegisterViewWithRegion method in your module's Initialize method. To illustrate this:

Constructor injection is supported by MEF. In order to achieve it, you must mark your constructor with the
ImportingConstructor attribute. You can read more about it in
this page from the
Mef Programming Guide.

I've modified my previous response to reflect that, since I had missed to include that in the code I had written. Sorry for the inconveniences.